There have been proposed various control methods and control devices to improve motion performance and safety performance of automobiles. Since motion such as running, turning and stopping of an automobile can only be operated via a friction force (grip force, tire force) between a tire and a road surface, detecting a force (load) acting on the tire can largely contribute to performance improvement related to automobile motion control. Therefore, a plurality of techniques related to detecting the force acting on the tire has been proposed.
Prior to the present invention, the applicant of the present invention previously considered a configuration of a vehicle wheel supporting rolling bearing unit having functions of supplying electric power to a sensor provided on a vehicle wheel (tire or wheel) and wirelessly communicating information acquired by the sensor to a vehicle body side (unpublished prior application: Japanese Patent Application No. 2015-186307). In the prior application, an electric generator is incorporated in a vehicle wheel supporting rolling bearing unit as a means for supplying electric power to a sensor. Further, in the prior application, handling of wiring can be eliminated completely when assembling the vehicle wheel supporting rolling bearing unit since a wireless communication device for wirelessly transmitting an output signal of a sensor is included. Patent document 1 describes an invention in which a power generation function and a power supply function are provided in a bearing unit to supply power to a sensor and perform radio communication.
However, when the electric generator provided in the bearing unit is used to supply electric power, following problems may occur in a case where a rotation speed of the vehicle wheel is low, that is, when a vehicle speed is low. In other words, since a generated voltage of the electric generator is generally proportional to the rotation speed of a rotor configuring the electric generator, it is difficult to ensure a generated voltage sufficient to operate the sensor and various circuits when the rotation speed is low. On the other hand, it is possible to increase a generated voltage at a low speed by increasing a number of coil turns of the electric generator, but even in such a case, there has to be a lower limit speed, and power cannot be generated when the vehicle is stopped. Further, since a rotation speed of a rotation side bearing ring member of the bearing unit for supporting the rotor largely varies according to a traveling speed of the vehicle, the generated voltage becomes undesirably excessive in high speed traveling when the generated voltage in low speed traveling is increased.
In order to solve the above problems, the inventors of the present application firstly considered to provide the bearing unit with a battery and a charge control circuit for the battery. In other words, in a case where a sufficient power generation amount cannot be obtained by the electric generator, such as in low speed traveling (including a case where the vehicle is stopped), a sensor and a wireless communication circuit are operated by electric power supplied from the battery. In a case where a sufficient power generation amount can be obtained by the electric generator, the sensor and the wireless communication circuit are operated and the battery is charged with the generated power. The charge control circuit controls the operations according to a generated voltage. According to the above configuration, the generated voltage is not excessive in high speed traveling, and electric power can be sufficiently supplied to the sensor and the wireless communication circuit even in low speed traveling.
However, in a case of simply adopting the configuration described above, there is a possibility that a large battery with a sufficient storage capacity has to be used according to a frequency of low speed traveling in a vehicle traveling time and power consumption of the sensor and the wireless communication circuit. The battery is advantageously as small as possible since it is disposed inside the bearing unit. Also, since a volume of the battery depends on the storage capacity, it is necessary to reduce a required storage capacity as much as possible so as to reduce the size of the battery.
Meanwhile, in consideration of using information on force (tire load) acting on a vehicle wheel acquired by the sensor for controlling motion of the vehicle, the necessity of the vehicle motion control is relatively lower in low speed traveling than high speed traveling. Also, when considering a traveling distance (running distance) of the vehicle, a time required for the vehicle to travel a certain distance increases as the vehicle speed decreases. Accordingly, low update frequency of the sensor information is acceptable (that is, a long update time interval is acceptable) when the vehicle speed is low.
Patent Documents 2 and 3 are other prior art related to the present invention. Patent Document 2 describes a technique in which a bearing unit includes a device for operating a sensor by contactless power supply, a power supply monitoring unit is provided for the device to monitor a received voltage and control the wireless transmission power according to the voltage value. However, the invention described in Patent Document 2 is directed to contactless power supply and controls transmission power. Patent Document 3 describes that power is supplied to a sensor provided on a vehicle wheel side by an electric generator or a battery, but using both the electric generator and the battery in combination is not described. Further, power required for wireless communication is also not described.